Because of their propensities to expand when contacted with water, various water swellable clays such as bentonite have been used in well plugging compositions for many years. However, they are typically introduced in solution, emulsion or other liquid/solid mixture with various chemicals, the selection of which is determined by the particular combination of substances used, the depth of the desired plugging location, the specific plugging objective, etc. For example Engle U.S. Pat. No. 3,407,878 discloses a method of sealing zones or formations of lost circulation of drilling fluids by separately introducing water/clay suspension, and an aqueous solution of a polyalkylene glycol polymer, which interact to flocculate and coagulate. In situations where it is desired to delay the coagulation process, he discloses the introduction of a barrier fluid after introduction of the water/clay suspension. Mintz U.S. Pat. No. 4,391,925 also discloses a similar approach to sealing zones in wells by introducing oil encapsulated bentonite and water, and then applying a shearing force to rupture the oil phase envelope to enable water to contact the bentonite to produce expansion and sealing. Drake U.S. Pat. No. 4,475,594 discloses a very similar process involving water-in-oil emulsion having bentonite dispersed in the oily phase which includes use of an oil soluble amine and a polyamine derivative surfactant. Armentrout U.S. Pat. No. 3,028,913 discloses a sealing composition consisting of a ready-mix composition which is mixed with water and then pumped into the well to the desired plugging location, along with a delayed action accelerator (sodium phosphates or various chlorides). More recently, Mason U.S. Pat. Nos. 4,463,808 and 4,566,708 disclose injecting a mixture of water, water swellable clay and a polymer (such as polyacrylamide), which inhibits swelling of the clay for a period of time, hopefully until the mixture reaches the desired location. All of these well plugging compositions are injected into the well in liquid form and require multiple components which must be transported and mixed and require the use of pumps or other equipment to introduce them into the well. They each include various chemical components which would be undesirable in drinking water wells or environmental monitoring wells. All of them recognize the need to prevent premature swelling and setting up of the clay and use various approaches to solve the problem. They require expensive grouting equipment and are labor intensive, in relation to applicant's invention.
In contrast, applicant has developed a composition consisting of compressed bentonite pellets or bentonite chips which are coated with a food grade white shellac which is water soluble. This composition may be introduced by pouring directly from a bag into the bore hole and allowing them to descend by gravity to the desired plugging location. Armentrout U.S. Pat. No. 2,836,555 discloses compressed bentonite pellets coated with a water insoluble material, and having small holes drilled into each pellet which permits the entry of an initial small quantity of water, and as more water is slowly absorbed eventually the pellets swell sufficiently to burst the coating and expose more of the bentonite to the water and the coagulation process is thus completed. Compared to applicant's invention, however, the pellets of Armentrout require an additional and expensive process step of drilling a hole into each pellet, and the chemical substances he discloses for coatings would be undesirable in water wells and monitoring wells.
As a demonstration of the difficulty presented to applicant in developing this invention, it may be noted that after testing many potential compositions which turned out to be unacceptable for a variety of reasons, applicant first settled on a composition of bentonite coated with natural resin in solution consisting of three pound pure white shellac in SDA1-2, 190.degree. ethyl alcohol. Although this composition was found to be effective in sealing wells, it gives off a strong alcohol odor which was offensive to product handlers, and it was discovered that it introduced undesirable residues of methyl isobutyl ketone (commonly referred to as MIBK) which were unacceptable in drinking water wells and monitoring wells. Only after continuing the development process for additional months was applicant able to come up with a product which is both functional and environmentally safe.
Although bentonite pellets have been used for years to plug wells, uncoated pellets have presented many problems, particularly when the location of the desired seal was below the water level and/or in narrow spaces between the wall of the bore hole and the well casing. Uncoated pellets begin to expand immediately upon becoming moist and the surface becomes sticky. When the pellets are dropped into a well and become moist before reaching the desired seal location, they begin to stick to the sides of the well casing and/or the bore hole, and may form bridges or clumps that prevent a sufficient number of pellets from reaching the desired seal location in the area beneath the bridge or clump, resulting in a nonuniform seal and in some cases an incomplete seal. The coated pellets of applicant's invention do not swell or become sticky until the coating is fully dissolved and thus do not bridge or clump. The coating is biodegradable and environmentally safe, and when dissolved exposes the bentonite which then expands to form a tough but flexible seal that will withstand considerable seismic movement and remain water impermeable. The coating on the pellets has an essentially uniform dissolve rate of 1 mil every three minutes when exposed to water, and thus the coating thickness can be varied depending upon the depth of the plugging location.